Ammonium sulphate is commonly produced as a crystalline product by continuous evaporative crystallization. The most common use for this product is as an agricultural fertilizer. It is commercially important that ammonium sulphate be blendable with other fertilizers since this is desirable in many application operations. However, the physical properties of typical ammonium sulphate crystals makes such a product unsuitable for blending due to poor sizing characteristics. Accordingly, a considerable amount of ammonium sulphate currently is produced as a screened crystal for use as a fertilizer in unblended form.
The production of ammonium sulphate granules, having characteristics (e.g. size) comparable to other conventional fertilizers, is highly desirable from a marketing point of view, because of the blending capability. In addition to proper sizing, such granules must be free-flowing, non-caking and stable. They must be sufficiently hard, i.e. non-friable, to overcome crumbling during storage and distribution, and must not be susceptible to ambient moisture absorption.
Various attempts have been made to 'overcome problems inherently associated with the production of crystalline ammonium sulphate by improving crystallization and nodulization processes, and by developing surface coating and granulation processes.
Processes for the production of ammonium sulphate, using various reactors such as pipe reactors, are described in the literature. The use of various rotary granulators, the crushing of oversize material, the recycling of fines and the addition of elements as a source of micronutrients, are known.
Rumscheidt et al., U.S. Pat. Nos. 2,043,066, 2,043,067 and 2,043,068 (June, 1936), describe the production of coarsely crystalline ammonium sulphate by adding water soluble salts to an aqueous solution of ammonium sulphate and evaporating the solution. Combinations of water soluble salts include those containing aluminum/manganese, aluminum/zinc and aluminum with one of sodium, potassium or magnesium.
The production of non-caking ammonium sulphate in the form of flat, scale-like crystals by adding aluminum sulphate to ammonium sulphate and maintaining a solution pH of 5.8-7.0 is disclosed in U.S. Pat. No. 2,092,073 by Jeltsch (September 1937).
Costolow U.S. Pat. No. 2,782,097 (February 1957) discloses a method for improving the production of ammonium sulphate crystals in a continuous evaporative crystallization process by extending the metastable region suitable for such crystallization in the presence of soluble trivalent ions of chromium, iron and aluminum.
U.S. Pat. No. 3,351,455 to Burns (November 1967) describes a method of making granular ammonium sulphate fertilizer by contacting a recirculating stream of granulation nuclei, comprising undersize particles of said product, with a sulphuric acid and ammonia reaction product. The product from the granulation zone has a bulk pH of less than 2.5. Attempts at raising the pH above 2.5 resulted in the production of an excessive amount of fines and dust with operational problems due to the equipment filling with dust.
U.S. Pat. No. 4,589,904 to Harrison et al. (May, 1986) details a process for granulating crystalline by-product ammonium sulphate using an ammonium sulphate/sulphuric acid/alum solution and gaseous anhydrous ammonia. The freshly precipitated ammonium sulphate produced binds the crystals together.
According to Japanese Patent 52006645, issued in 1977, a sulphate of a cation of a minor nutrient can be calcined with ammonium sulphate to give an anhydrous salt which is compounded with powdery or granular fertilizers. A sulphate of zinc can be such a material. This addition maintains good fertilizer flow properties during storage.
According to a process for the commercial production of granular ammonium sulphate using a basic process described in Australian Patent 492,758, feed materials, including sulphuric acid and ammonia, are reacted in the presence of appropriate amounts of an aluminum salt granulating aid. The reaction product is discharged onto a bed of recycled fines in a rotary granulator. The granulated material is screened with on-size material being the product. Oversize material is crushed and recycled along with undersize fines to the granulation step. More particularly, ammonium sulphate is granulated in the presence of a granulation aid which is an aluminum or ferric salt to produce a granulation product with a pH of 4.0 to 4.5.
PCT Patent Application W089/04291, published May 18, 1989, describes a method for producing free-flowing, non-caking granular ammonium sulphate having a pH between 2.5 and 4.0 in the presence of a granulating aid selected from the group consisting of a metal salt, a metal oxide, and a salt of a metal hydroxide. The metal salt is selected from the group consisting of aluminum and ferric salts, as in Australian Patent 492,758, and preferably is aluminum salt. The process described differs from the basic process in the original Australian patent in that the granulation is conducted at a pH range of 2.5 to 4.0 which previously was considered unusable.
In U.S. Pat. No. 5,043,007 to Davis, (August 1991), a process is disclosed for coating fertilizer particles, such as ammonium sulphate, with a mixture of nutrients and metallic salts, such as hydrated zinc sulphate.
In the production of zinc, by the electrolytic process, a bleed of a portion of zinc electrolyte is necessary in the control of the process. This is a requirement of all such similar operations worldwide and is needed to remove excess water (solution bulk) and sulphate, as well as deleterious elements, mainly magnesium and halides, which are detrimental to the electrolytic zinc process and which are not removed in the normal electrolyte purification process.
Various methods are practised in the handling and processing of this purge. One such processing scheme is electrolysis of the bleed stream in order to strip the solution of its zinc content, although a significant soluble zinc content remains in the stripped zinc electrolyte. The stripped solution also contains sulphuric acid and dissolved manganese. The stripped solution may then be neutralized. Table 1 shows a typical chemical analysis of the stripped zinc electrolyte in an electrolytic plant.
TABLE 1 ______________________________________ TYPICAL COMPOSITION OF STRIPPED ZINC ELECTROLYTE ______________________________________ zinc 20-25 g/L magnesium 2-4 g/L manganese 1-2 g/L sulphuric acid 190-200 g/L total sulphate 250 g/L calcium 400 mg/L ______________________________________
It has now been surprisingly found that such stripped zinc electrolyte can be used as an effective granulating aid in the production of granular ammonium sulphate fertilizer, eliminating the use of conventional granulating aids, while providing valuable micronutrients to the fertilizer.